1. Field of the Invention
The present invention relates to the design of a supercritical pressure once-through boiler system for load cycling and two-shift on/off cycling.
A trend has developed in the electric power industry toward cycling operation of large fossil-fired boiler units, in contrast to the former utility practice of base loading these larger, more efficient units. This trend has led to design problems involving thermal stresses and their corresponding effects on fatigue life. When a supercritical pressure boiler unit is idle after a brief shut down, the pressure decays to subcritical levels. In such units designed and manufactured by The Babcock & Wilcox Company, furnace pressure must be restored to supercritical levels by pumping and minimum flow in the furnace circuits established before firing can be resumed. Since under these conditions only relatively cold feedwater is available from a once-through unit, its use thermally shocks pressure parts with each restart. As a result, the unit is not suitable for the on/off cycling service required by this trend.
2. Description of the Related Art
Stevens, et al (U.S. Pat. No. 3,954,087) discloses a steam power plant start-up system and method including vertical separators located in the main flow path upstream of the superheaters. During start-up, separator liquid flows in an auxiliary flow circuit to the condenser or the main flow path upstream of the economizer. Vapor from the separators flows directly into the main flow path to the superheater and turbine. The system operates at variable pressure during start-up and supercritical pressure during operation. Stevens, et al (U.S. Pat. No. 4,099,384) is similar to the system of U.S. Pat. No. 3,954,087 except for the addition of a pressure control station upstream of the separators to maintain supercritical pressure in the furnace during start-up. Gorzegno (U.S. Pat. No. 4,241,585) describes a method of operating the vapor generator disclosed in Stevens, et al (U.S. Pat. No. 4,099,384), in which full turbine throttle pressure is reached at a higher load than that disclosed in the aforementioned U.S. Pat. No. 4,099,384 Stevens, et al patent. Miszak (U.S. Pat. No. 4,430,962) discloses a steam generating plant which is similar to the U.S. Pat. No. 3,954,087 Stevens, et al patent. The system includes a vertical separator upstream of the superheaters, and operates at variable pressure at partial loads and preferably at supercritical pressure at full load.